An idiomatic expression meaning roughly "to explode in a fit of rage and fury", it is derived from the action of a firearm hammer "dropping" -- or striking -- the firing pin which in turn impacts the round and causes the mini explosion that results in a bullet or other projectile being launched at a high velocity out of the end of a barrel.

The expression can be used in several different variations with the above definition: 1) ...part. in a draconian way 2) ...leaving a wake of destruction in its path 3) ...against a very specific target with precision; etc.

"I have a clear shot of the target. Requesting permission to engage."
"You have the green light. Drop the hammer on that motherfucker."
BANG. Bad guy fall down.